The present invention relates to a method of determining nitrate concentration in water and to a catalyst for this purpose which can reduce nitrates selectively in water.
During the last few years, the concentration of nitrate in surface water and ground water has continuously increased as a result of, e.g., overfertilization in agriculture. It can be noted from the Chemical Water Statistics of the Waterworks of the Federal Republic of Germany in West Berlin (edited by Dr. Gerhard Giebler, published by R. Oldenbourg, Munich 1959) that the nitrate content in ground and surface waters is in most cases 50 mg of nitrate per liter, or even more. According to the World Health Organization, the nitrate content of drinking water should not exceed 50 mg/l since with such high nitrate content there is the danger of methemoglobinemia in children. In accordance with recent findings, nitrate in combination with certain amines can form nitrosamine in the digestive tract, which substance must be considered strongly carcinogenic. It is thus necessary continuously to monitor the nitrate content of the water.
Numerous methods for determining nitrate concentration in water are known. Most determinations are based on color reactions which include either a reduction of the nitrate to nitrite, followed by diazotization to form azo dyes, or a direct reaction of the nitrate with p-fluorophenol. This last-mentioned photometric determination by fluorophenol has been introduced into the German Standardization Methods. However, this determination is very expensive and requires careful monitoring of the dosaging of the chemicals, and this method is poorly suited for a rapid continuous determination of nitrate in water.
Nitrate can also be determined by direct reaction with brucine. This method of determination, however, can be carried out only under very strict precautionary measures since brucine is a strong poison. Such a method is unsuitable for the continuous monitoring of nitrate in water.
During the last few years, ion-sensitive electrodes have also become known which permit selective determination of the nitrate ion in water. Thus, the journal Fresenius Z. Anal, Chem. 297, pages 414 and 416 (1979), describes a nitrate-ion-selective electrode having a base of cuprous neocuproine complex. However, it has been found that such ion-selective electrodes are susceptible to failure to a considerable extent so that they cannot be recommended for the continuous determination of nitrate in water.
It is also known that nitrate shows a pronounced absorption maximum in the UV region at 210 nm, which maximum could be used in general for a quantitative determination of nitrate. Since, however, surface, ground and waste water contain a large number of inorganic and organic substances which absorb in the UV region and are thus superimposed on the nitrate absorption, no method has become known up to now for a reproducible determination of nitrate via UV absorption.
The object of the present invention is, accordingly, to provide a method for determination of nitrate in water and a catalyst which can selectively reduce nitrate without at the same time reducing other entities present in the water, so that a rapid and sensitive measurement of the nitrate in water is possible by UV absorption, particularly in continuous operation.
The object of the invention is achieved by providing a catalyst for the determination of nitrate in water which is characterized by the fact that it has been obtained by reduction of a solution of a salt of a transition metal of Group VIII of the periodic system and of a transition metal salt of the copper group in the presence of an acid.
The catalyst of the invention is capable of selectively reducing nitrate in water in the presence of hydrogen, without sulfate, for instance, being reduced to sulfide, which would poison the catalyst. The catalyst can be used for the determination of nitrate in tap, ground, surface and waste waters.
The catalyst of the invention will be described in detail below.
The transition metal of Group VIII of the periodic system is preferably a metal of the platinum group, especially platinum or palladium. For the preparation of the catalyst, (M).sub.2 PtX.sub.6, (M).sub.2 PdX.sub.6, PdX.sub.2 or PtX.sub.2 is preferably used as transition metal salt of the platinum group, M being an alkali metal, ammonium or hydrogen and X being halogen. Ammonium hexachloroplatinate is a particularly suitable transition metal salt.
As salt of the copper group, there can be used an carbonate, nitrate or halide. Also the oxide can be used.
In the preparation of the catalyst of the invention, it is important that the transition metal of Group VIII of the periodic system and the transition metal of the copper group be used in a particular molar ratio. In preliminary investigations, it has been found that while nitrate is reduced by Raney nickel and also by platinum, nevertheless the sulfate present in the water was reduced to sulfide. Since sulfide is a catalyst poison, the Raney nickel catalyst soon becomes inactive so that it is unsuitable for the present purpose. Similar results were obtained in preliminary experiments with platinum, the platinum being rapidly inactivated even in the presence of only 10 mg/l of sulfate.
For the above reasons, the transition metal of Group VIII of the periodic system and the transition metal of the copper group are present in the catalyst in a molar ratio range of 1:8 to 8:1 and preferably 1:4 to 4:1. The molar ratio range of the transition metal of Group VIII of the periodic system to the transition metal of the copper group is, most preferably, about 1:2 to 1:4.
In the reduction of the transition metal salts to the catalyst of the invention, the acid used is generally an inorganic acid, particularly hydrochloric acid or nitric acid, or a mixture of these two acids. Instead of the direct use of transition metal salts, the transition metals can also be used in elementary form by, for instance, dissolving platinum shavings and copper wire in aqua regia.
Upon the preparation of the catalyst, the reduction of the transition metal salts takes place simultaneously, the reducing agent used being one which lies below copper in the electrochemical displacement series. Preferred reducing agents are, for instance, zinc dust or a hydrazine compound.
The catalyst is obtained by reduction at a temperature of 10.degree.-150.degree. C., and particularly 40.degree.-80.degree. C.
In accordance with a preferred embodiment, the solution of transition metal salts is applied to a support material and the reduction being carried out in the presence of the support. The solution of the transition metal salts is advisedly first subjected to a vacuum in the presence of a porous support so that the support is impregnated with the solution. This process step can be carried out, for instance, in a vacuum dessicator. Thereupon, the solution with the impregnated support material is brought back to atmospheric pressure and the transition metal salts introduced are then reduced.
As porous support materials, ceramic materials such as alumina, magnesia, silica gel or kieselguhr, activated charcoal or expanded clay enter into consideration. The support can be used in the form of Berl saddles or Raschig rings so that the catalyst applied to the support can be used in packed columns.
In accordance with another embodiment, the catalyst can also be present in the form of a coating on an electrode which is required for the liberation of hydrogen. In this case, the electrode may consist of any suitable material, for instance, carbon.
The method of the invention for the determination of nitrate in water is characterized by the fact that the nitrate extinction of a sample of water is determined as compared with a blank sample in the UV region at 210 nm, the blank sample being a part of the sample of water which is reduced in the presence of hydrogen with the use of the catalyst of the invention. The method of the invention permits a rapid, selective determination of the nitrate in water, without any substantial manipulating of the sample of water being necessary, and it is possible to avoid any substantial addition of chemicals. The method of the invention is particularly suitable for the continuous determination of nitrate and thus for the continuous monitoring of the quality of the water.
The method of the invention will be explained in further detail with reference to FIGS. 1 to 3 of the accompanying drawings.